Cities looking for ways to reduce flooding could do so by planting more trees, suggests a University of Maryland study published in Scientific Reports. The researchers found that urban trees–even single trees standing in a park or lining a city street–can capture and store enough rainwater to substantially aid stormwater management.
Trees in any setting consume water and eventually release it back into the atmosphere as water vapor, in the process of “transpiration.” But the transpiration process occurs three times as quickly in individually planted trees compared to trees in a forest, according to the Maryland researchers, who included Assistant Professor Mitch Pavao-Zuckerman and doctoral candidate Sara Ponte, both of the Department of Environmental Science and Technology. The researchers suggested that cities combating excess rainfall and snow melt could lower the water flows by planting more trees in city areas.
Pavao-Zuckerman and Ponte partnered with the U.S. Forest Service and the nonprofit Center for Watershed Protection, and received funding from the Chesapeake Bay Trust. The two researchers measured transpiration among two groups of trees in Montgomery County, Maryland–single trees over turfgrass and a cluster of trees over turfgrass–and a closed canopy forest in Baltimore. They built sensors and implanted them onto the trees to track how the trees access groundwater as well as their transpiration rates, the soil water content, and the air temperature, humidity, and rainfall at all three sites.
“Quantifying the impacts of urban trees… such as the evapotranspiration component discussed in Mitch and Sarah’s paper, gives us a better understanding of the benefits of urban trees, and knowing where and how to plant and preserve them to achieve the greatest benefit.” said Deb Caraco, senior watershed engineer with the Center for Watershed Protection.
Transpiration is a well-known process in the scientific community, and urban planners have considered trees’ potential to mitigate stormwater flooding before. But current research on trees and stormwater flows have been based on forest trees’ comparatively slower rates of transpiration, not freestanding urban trees, and therefore do not paint a fully accurate picture, Pavao-Zuckeman said.
Pavo-Zuckerman said that more research will be necessary to determine how stands or patches of trees might impact stormwater flows in cities. The findings would help urban planners devise plans that positively impact stormwater management in their neighborhoods.
“Our data can help make tree-crediting policies better reflect the actual benefits of trees in urban landscapes, because they interact with water and their environment differently in cities than they do outside cities,” he said.